Feature: Add boats to ZTM City (map_city)

In map_city we have a lot of water because ZTM City is placed in the middle of a sea side.

Add some boats which can just sit inside of the water for now. We will add more movements and interactions in another issue once we know how many we have. It would be nice if they have some animation around it. Boats usually move slightly up and down even if they don't move because the water is constantly moving.

Feel free to search for free to use sprites. Make sure to add them to the asset_credits.md file to give a shoutout to the creators of the sprites which you have used.